1. Field of the Invention
The field of the present invention pertains to diagnostic x-ray imaging equipment. More particularly, the present invention pertains to real-time scanning-beam x-ray imaging systems.
2. Description of Related Art
Real-time x-ray imaging is increasingly being required by medical procedures as therapeutic technologies advance. For example, many electro-physiologic cardiac procedures, peripheral vascular procedures, PTCA procedures (percutaneous transluminal catheter angioplasty), urological procedures, and orthopedic procedures rely on real-time x-ray imaging. In addition, modern medical procedures often require the use of instruments, such as catheters, that are inserted into the human body. These medical procedures often require the ability to discern the exact location of instruments that are inserted within the human body, often in conjunction with an accurate image of the surrounding body through the use of x-ray imaging.
Current clinical real-time x-ray equipment produces high levels of x-ray exposure to both patients and attending staff. The United States Food and Drug Administration (F.D.A.) has reported anecdotal evidence of acute radiation sickness in patients, and concern among physicians of excessive occupational exposure. (Radiological Health Bulletin, Vol. XXV1, No. 8, August 1992).
A number of real-time x-ray imaging systems are known. These include fluoroscope-based systems where x-rays are projected into an object to be x-rayed and shadows caused by relatively x-ray opaque matter within the object are displayed on the fluoroscope located on the opposite side of the object from the x-ray source. Scanning x-ray tubes have been known in conjunction with the fluoroscopy art since at least the early 1950s. Moon, Amplifying and Intensifying the Fluoroscopic Image by Means of a Scanning X-ray Tube, Science, Oct. 6, 1950, pp. 389-395.
Reverse-geometry scanning-beam x-ray imaging systems are also known. In such systems, an x-ray tube is employed to generate x-ray radiation. Within the x-ray tube, an electron beam is generated and focussed upon a small spot on the relatively large anode (transmission target) of the tube, inducing x-ray radiation emission from that spot. The electron beam is deflected (electromagnetically or electrostatically) in a raster scan pattern over the anode target. A small x-ray detector is placed at a distance from the anode target of the x-ray tube. The detector typically converts x-rays which strike it into an electrical signal in proportion to the detected x-ray flux. When an object is placed between the x-ray tube and the detector, x-rays are attenuated and scattered by the object in proportion to the x-ray density of the object. While the x-ray tube is in the scanning mode, the signal from the detector is inversely proportional to the x-ray density of the object.
Examples of known reverse-geometry scanning-beam x-ray systems include those described in U.S. Pat. No. 3,949,229 to Albert; U.S. Pat. No. 4,032,787 to Albert; U.S. Pat. No. 4,057,745 to Albert; U.S. Pat. No. 4,144,457 to Albert; U.S. Pat. No. 4,149,076 to Albert; U.S. Pat. No. 4,196,351 to Albert; U.S. Pat. No. 4,259,582 to Albert; U.S. Pat. No. 4,259,583 to Albert; U.S. Pat. No. 4,288,697 to Albert; U.S. Pat. No. 4,321,473 to Albert; U.S. Pat. No. 4,323,779 to Albert; U.S. Pat. No. 4,465,540 to Albert; U.S. Pat. No. 4,519,092 to Albert; and U.S. Pat. No. 4,730,350 to Albert.
In a typical known embodiment of a reverse-geometry scanning-beam system, an output signal from the detector is applied to the z-axis (luminance) input of a video monitor. This signal modulates the brightness of the viewing screen. The x and y inputs to the video monitor are typically derived from the signal that effects deflection of the electron beam of the x-ray tube. Therefore, the luminance of a point on the viewing screen is inversely proportional to the absorption of x-rays passing from the source, through the object, to the detector.
Medical x-ray systems are usually operated at the lowest possible x-ray exposure level at the entrance of the patient that is consistent with the image quality requirements (particularly contrast resolution and spatial resolution requirements) for the procedure and the system. Typical patient entrance exposure in conventional 9" field of view image intensifier systems used in cardiac procedures, in the AP (anterior posterior) view with a standard adult chest, is approximately 2.0 to 2.8 R/min. The term "low dosage" used herein refers to a factor of 2 to 20 less than this.
Time and area distributions of x-ray flux follow a Poisson distribution and have an associated randomness which is unavoidable. The randomness is typically expressed as the standard deviation of the mean flux, and equals its square root. The signal-to-noise ratio of an x-ray image under these conditions is equal to the mean flux divided by the square root of the mean flux. i.e., for a mean flux of 100 photons, the noise is .+-.10 photons, and the signal-to-noise ratio is 10.
Accordingly, the spatial resolution and the signal-to-noise ratio of x-ray images formed by known reverse-geometry scanning x-ray imaging systems are dependent, to a large extent, upon the size of the sensitive area of the detector. If the detector aperture is increased in area, more of the diverging rays are detected, effectively increasing sensitivity and improving the signal-to-noise ratio. At the same time, however, the larger detector aperture reduces attainable spatial resolution as the "pixel" size (measured at the plane of the object to be imaged) becomes larger. This is necessarily so because most objects to be imaged in medical applications (e.g., structures internal to the human body) are some distance from the x-ray source. In the known systems, therefore, the detector aperture size has been selected so as to effect a compromise between resolution and sensitivity, it not being previously possible to maximize both resolution and sensitivity simultaneously.
In the medical field, several conflicting factors, among them patient dosage, frame rate (the number of times per second that the object is scanned and the image refreshed), and resolution of the image of the object, often work to limit the usefulness of an x-ray imaging system. For example, a high x-ray flux may easily yield high resolution and a high frame rate, yet result in an unacceptably high x-ray dosage to the patient and attending staff.
Similarly, lower dosages may be achieved from the known systems at the cost of a low resolution image or an inadequate refresh rate. A preferred medical imaging system should provide low patient dosage, high resolution and an adequate refresh rate of up to at least about 15 images per second--all at the same time. Therefore, systems such as the known reverse-geometry scanning-beam x-ray imaging systems described above are not acceptable for diagnostic medical procedures where exposure times are relatively long and where, as is always the case with live patients, the x-ray dose received by the patient should be kept to a minimum.
Minimally invasive procedures in medicine are typically characterized by access to areas inside the body using existing orifices such as the ureter or by percutaneous entry such as a puncture of the femoral vein. In such procedures, various tools and catheters may then be progressed into the body and maneuvered using a real-time x-ray imaging system for guidance. An estimated 3,000,000 medical procedures of this type were performed in 1993 under x-ray fluoroscopy guidance. Many of these procedures involve the introduction of a catheter into the coronary arteries and the heart, and the evaluation of cardiac function by inspection of images taken when contrast media is introduced via a lumen in the catheter. Some of the tools that may be inserted in this manner include lasers where the laser device is located outside the body and the laser light delivered to the site of interest with a fiber-optic wave guide disposed in a catheter, drug delivery systems adapted to deliver precisely measured quantities of a specific drug or radiological material to the site of interest, ultrasound systems in which a transducer on the tip is used to view a site of interest by delivering the image over to a video system which can then display and record images of the site of interest, and other tools known to the art. It is also possible to adapt such procedures to non-medical applications where access is difficult and the value of the procedure high, e.g., engine diagnosis and repair.
As used herein, the term "maneuverable positioner" is meant to collectively include and refer to, for example, catheters, probes, endoscopes, and other maneuverable positioners and tools.
The known medical x-ray imaging devices do not provide a highly-accurate determination of location for maneuverable positioners with a precise image of the patient's internal structure. Generally, the physician using known systems can roughly ascertain the position of maneuverable positioners relative to body features within the patient, but precision and repeatability, the ability to return to the exact same place, especially in the axis parallel to the x-ray beam, is lacking. Thus the distance between the x-ray emitting source and the maneuverable positioner within the body may not be readily or accurately determined with the precision useful in today's advanced medical procedures, which may require, among other things, the ability to determine a position with the maneuverable positioner, move the maneuverable positioner, and return the maneuverable positioner to the exact same place.
For example, since 1982 there has been increasing use of catheter ablation to cure certain types of arrhythmia. In these types of arrhythmia, such as Wolff-Parkinson-White syndrome, the conductive congenital muscle fibers can be made nonconductive by heating them locally to a sufficient temperature to cause scar tissue to form. Most of these ablations are done with radio-frequency energy but the emitting electrode must be placed within one to three millimeters of the muscle fiber location and it must stay in intimate contact with it for a number of heartbeats and respiratory cycles.
Although the treatment of arrhythmia through catheter ablation has some advantages, there are also some problems. The advantages of the procedure are that it has a very high success rate, it is minimally invasive, it can be performed in a few hours in a procedure room, and it is considerably less expensive than open chest surgery or a lifetime of drug therapy. The major disadvantage is that the length of the procedure is uncertain and typically long. This leads to difficulty in scheduling physicians and facilities, fatigue for both patient and staff, and high-radiation dosages for patient and physician.
Attempts to solve these problems have focused mainly on providing more steerable catheters to reduce the time to find the precise location of the ablation site and to position the catheter for remaining in contact with the substrate during the ablation time, which is typically five to ninety seconds. Having more steerable catheters has not yet reduced the time or uncertainty of time because the location of the catheter is generally determined by looking at an x-ray image projected on a monitor and by analyzing the electrocardiogram. Both of these actions must be done in real time in order to know whether to move the catheter and in which direction to move it. The actual direction of movement may be uncertain due to the nature of an x-ray image of soft tissue and blood, the poor control and feedback of the catheter, the movement of the heart, and the difficulty of determining direction from the electrocardiogram analysis.
In the U.S., there are currently 300,000 to 500,000 people who die each year due to arrhythmia that is a result of a myocardial infarction. However, it is believed that if the slow-conduction zone around the infarct could be electrically mapped and selectively ablated, that a cure could be obtained. Tests on animals and some humans have demonstrated the possibility of such a procedure but the success rate has been low. The reason for the low success is thought to be the need to map the entire area of the infarct and slow conduction zone and then to be able to ablate multiple sites without depending on acquiring a characteristic electrogram once the ablation has begun. Current investigations attempting to solve the problem utilizing a catheter network array of nodes suffers from the problem of extracting the catheter network array from inside the heart without damaging the internal structure of the heart.
For various reasons, the imaging modalities of MRI, CT, and ultrasound are not normally suitable when anatomical markers are needed during cardiac diagnostic and treatment procedures. In addition, the use of known methods employing x-ray fluoroscopes for imaging typically has the serious disadvantage of not being able to distinguish anatomical detail inside the heart. The physician relies on the shadows generated, his or her intimate knowledge of the anatomy, the characteristic movement of the image and catheters caused by the cardiac cycle and the respiratory cycle, and for fine positioning, the electrocardiogram.
Accordingly, there is a need for devices and methods to provide a precise determination of the coordinates of a maneuverable positioner within a human patient during a medical procedure. The same techniques and apparatus can also be used to advantage in any x-ray procedure which requires accurate determination of the X, Y and Z coordinates of the position of a maneuverable positioner which may be adapted to sense x-rays.